Model Insect Cuticle Sclerotization: Reactions of Catecholamine Quinones with the Nitrogen-Centered Nucleophiles Imidazole andN-Acetylhistidine

The catecholaminesN-acetyldopamine (NADA) andN-β-alanyldopamine (NBAD) are two precursors for quinonoios used as sclerotizing agents in insect cuticle. This study focused on the reaction pathways of the quinones of NADA and NBAD by using two nitrogen-centered nucleophiles, imidazole andN-acetylhistidine, to model cuticular proteins containing histidyl residues. The quinones were prepared by electrochemical oxidation, using either a coulometric microcell or a flow-through cell. The reactions of the quinones with the nucleophiles were investigated at physiological pH using electrochemical, chromatographic, and spectroscopic methods. The major products were purified by semipreparative liquid chromatography and identified by mass spectrometry and nuclear magnetic resonance spectroscopy to be nucleophilic addition products of the quinones with the nucleophiles bonded to two carbons in the aromatic ring. The predominant products for both nucleophiles were C6 adducts of NADA and NBAD. C2 adducts ofN-acetylhistidine were minor products.